Parliament has become so abused an institution — the playground squabbling, the grubby scandals — that a moment like last night’s vote comes as a shock. Seasoned commentators were reduced to tweeting “Crikey” and “Erm … Bloody hell”. But we should be clear: the Government’s defeat over British intervention in Syria could be a positive turning point for the kind of nation we are.

The paradox is that while David Cameron made a far better, more cogent speech than Ed Miliband’s plodding effort, and had the fundamental decency to make clear that he accepted Parliament’s verdict, he is likely now to suffer the more serious political damage.

Cameron deserves this to the extent that he has mismanaged his own MPs: you can’t imagine Tony Blair making a callow political miscalculation of the magnitude that Cameron has this week. Yet in the end, the sticking point was that Tory MPs and voters simply didn’t believe the Prime Minister. And for that, he has Blair to thank.

Because this was the week when it dawned on the political class just how long a shadow Iraq still casts: how the 10 years marked last spring since that war’s beginning had not, in fact, been long enough to judge its impact on our democracy.

Then, Blair had far longer to win over the nation and MPs than did Cameron. Over the course of more than a year ahead of the invasion, Blair lined Britain up behind a White House intent from the start on regime change.

The sticking point was that Tory MPs and voters simply didn’t believe the PM. And for that, he has Blair to thank

I was Blair’s civil service speechwriter for some of that period: in 2002, I watched aghast and depressed as we drifted towards war. From Blair’s visit to Crawford, Texas in April 2002, war seemed to me the likely outcome; from that August, inevitable.

Then we had two dodgy dossiers of “intelligence”: the public remained unconvinced. Less than two weeks after the release of the second, I joined the London march of February 15 2003 (I had left Downing Street by then), my 18-month-old daughter wrapped up on my back, two of us among perhaps two million very middle-English people.

And then Parliament had its say. Blair’s speech on March 18 2003 was a masterpiece: impassioned, its logic laser-like. It was the life-or-death gamble of a political genius — and he won. It was also deeply disingenuous and committed us to a disastrous war. Ten years and hundreds of thousands of deaths later, most of the predictions of the war’s opponents have proved correct. The Middle East has become more violent and unstable. Terrorism has spread rather than ended.

That is why Cameron’s own supporters — much less Labour — did not believe his eloquence. As Jack Straw said with some humility yesterday, Iraq “raised the bar” for the level of proof Britons now require to go to war — perhaps to a level that can almost never be satisfied.

But if that means that we no longer march in lock step as the US’s little helper, I applaud it. Tony Blair was both lionised and reviled for taking the supposedly difficult, unpopular decision over Iraq. It may have been those things — but it was still the default position of British foreign policy since Suez, the line of least resistance.

The far harder path, of trying to carve out a moral world role for ourselves not dictated by the US, is the one the Commons voted for last night. And frankly, today that makes me feel proud to be British.